update documentation for IsValid()
update documentation for IsValid()

see https://savannah.cern.ch/bugs/index.php?96316

git-svn-id: acec3fd5b7ea1eb9e79d6329d318e8118ee2e14f@45816 27541ba8-7e3a-0410-8455-c3a389f83636

tc3t/qoot,kirbyherm/root-r-tools,tc3t/qoot,tc3t/qoot,kirbyherm/root-r-tools,tc3t/qoot,tc3t/qoot,tc3t/qoot,kirbyherm/root-r-tools,tc3t/qoot,tc3t/qoot,tc3t/qoot,kirbyherm/root-r-tools,tc3t/qoot,kirbyherm/root-r-tools,kirbyherm/root-r-tools,kirbyherm/root-r-tools